Ultra-wideband (UWB) communications technology has gained attention in recent years as one of high-speed radio transmission methods. The UWB communications technology enables communications over an extremely broad bandwidth by using pulse trains made up of pulse signals synchronized with particular periodic timing. Known as one form of UWB communications is communication carried out by using pulse trains made up of pulse signals of extremely short pulselengths, such as 1 ns or less, without using a carrier (refer to Patent Documents 1 and 2, for example).
FIG. 18 is a block diagram showing a radio receiving apparatus 1000 for UWB communications according to the prior art. The radio receiving apparatus 1000 shown in FIG. 18 is provided with an antenna 1001 for receiving a UWB communications signal sent from a radio transmitting apparatus by UWB communications, an amplifier 1002 for amplifying the UWB communications signal received by the antenna 1001, an integration circuit 1003 for integrating a signal Sy amplified by the amplifier 1002 based on a template signal Sx indicating specific timing in a time sequence output from a controller 1005 to thereby generate an integral voltage Sz indicating a correlation between the template signal Sx and the signal Sy, an analog-to-digital converter (hereinafter referred to as “A/D converter” for short) 1004 for converting the integral voltage Sz into a digital value, and the controller 1005 for outputting the template signal Sx to the integration circuit 1003 while synchronizing the same with the UWB communications signal based on correlation values obtained by the A/D converter 1004 and for performing demodulation to extract data from the correlation values obtained by the A/D converter 1004.
The radio receiving apparatus 1000 thus configured periodically generates the template signal Sx in synchronism with UWB communications pulse signals and receives the UWB communications pulse signals during specific periods indicated by the template signal Sx, such as during 10-ns window periods only, whereby the radio receiving apparatus 1000 receives significant UWB communications pulse signals by excluding background noise which has no fixed periodicity like the UWB communications pulse signals.
UWB communications use a pulse signal of extremely short pulselengths, such as 1 ns or less, without using a carrier. Therefore, spectral density of transmitted power in UWB communications is extremely low as compared to other radio communications methods. For this reason, internal circuits including the amplifier 1002, the integration circuit 1003, the A/D converter 1004 and the controller 1005 are configured to achieve high sensitivity and, thus, there could arise a situation in which these internal circuits pick up noise as a signal. In particular, noise that occurs due to clock signals used in these internal circuits, such as a reference clock signal used in the controller 1005 for generating the template signal Sx, has periodicity. Therefore, there can occur a case where noise is misinterpreted as a received pulse, causing such inconvenience as a reduction in reliability of communication, if such noise having periodicity becomes synchronized with window periods indicated by the template signal Sx.
Patent Document 3 shows an arrangement for searching for a synchronization signal pattern and achieving synchronization when synchronizing receive timing of a radio receiving apparatus with a pulse signal made of a pulse train, wherein a search for the synchronization signal pattern is carried out by increasing amplifier gain when a received signal pattern (pulse train) always has a “0” value on the assumption that the gain is too low, and decreasing the amplifier gain when the received signal pattern always has a “1” value on the assumption that the gain is too high.
The prior art of Patent Document 3 employs an amplifier gain approach. This approach is preferable for optimizing a noise floor level due to thermal noise of the radio receiving apparatus, for instance, relative to a received signal level and extracting the aforementioned pulse train. If there exists an interfering wave within a receiver bandwidth, and especially if power of the interfering wave is large, it becomes impossible to receive signals.    Patent Document 1: Japanese Unexamined Patent Publication No. 2005-217899    Patent Document 2: Japanese Unexamined Patent Publication No. 1998-508725    Patent Document 3: Japanese Unexamined Patent Publication No. 2006-94169